1. Field of the Invention
The invention relates generally to audio amplification systems and more particularly to systems and methods for modifying the timing of one or more channels of an audio amplification system to stagger switching noise in the channels and thereby modify the character of the noise.
2. Related Art
Pulse Width Modulation (PWM) or Class D signal amplification technology has existed for a number of years. PWM technology has become more popular with the proliferation of Switched Mode Power Supplies (SMPS). Since this technology emerged, there has been an increased interest in applying PWM techniques in signal amplification applications as a result of the significant efficiency improvement that can be realized through the use of Class D power output topology instead of the legacy (linear Class AB) power output topology.
Early attempts to develop signal amplification applications utilized the same approach to amplification that was being used in the early SMPS. More particularly, these attempts utilized analog modulation schemes that resulted in low performance applications. These applications were complex and costly to implement. Consequently, these solutions were not widely accepted. Class D technology was therefore unable to displace legacy Class AB amplifiers in mainstream amplifier applications.
Recently, digital PWM modulation schemes have surfaced. These schemes use Sigma-Delta modulation techniques to generate the PWM signals used in the newer digital Class D implementations. These digital PWM schemes, however, did little to offset the major barriers to integration of PWM modulators into the total amplifier solution. Class D technology has therefore continued to be unable to displace legacy Class AB amplifiers in mainstream applications.
There are a number of problems with existing digital PWM modulation schemes. One of the problems is that audio system implementations are requiring increasing numbers of channels. For example, a home theater audio system may need to provide not only a single pair of stereo channels, but also a second pair of stereo channels (for a pair of front speakers and a pair of back speakers) and a center channel (e.g., for a sub-woofer speaker). Another example of a system that may require a large number of channels is a system which is intended to provide audio to a large area, such as multiple rooms within a building. Implementations such as these require more channels than are typically provided in a digital PWM system.
Existing digital PWM amplification systems only have as many channels as can be implemented on a single chip. Typically, these systems have either two or four channels. While it is possible to provide additional channels on a single chip, this typically is not a practical solution for several reasons. For example, there simply may not be enough space on the chip to implement the additional channels. It may also be possible that there are not enough resources (e.g., processor cycles) to process all of the channels on the same chip. Further, the complexity of the design may increase dramatically with the additional channels. Still further, even if a few additional channels could be accommodated, such a solution would not address the next generation of system requirements in which still more channels were required.
Existing digital PWM systems are not implemented across multiple chips because of difficulties that are associated with the interaction of multiple chips. One such difficulty may be the problem of synchronization. In order for the system to provide coherent control of all of the channels in the system, it is necessary to synchronize each of the chips so that they operate essentially as if the system were implemented on a single chip. No such mechanism currently exists for digital PWM audio amplification systems. Another problem is that, once the chips are synchronized, if the data content of the channels is highly correlated, all of the channels are switched almost simultaneously. This is problematic because the switching causes noise in the audio signal, and the near-simultaneous switching of all of the channels increases the noise level. It should be noted that this switching noise occurs in single-chip systems, as well as multi-chip systems.